This chapter concerns vulnerability as a constitutive feature of human agency and argues that the ontological approach to vulnerability provides an important insight about rational agency and practical reasoning. This claim is defended in contrast to two established theoretical approaches to vulnerability. On the one hand, most theories of rationality are primarily concerned with vulnerability as a source of limitations and defects, and they presume to offer normative guidance by adopting an idealized account of rational agency, which brackets the defective features of human agency. On the other hand, theories of bounded or limited rationality and theories of minimal agency reject idealization; and theories of impure agency privilege a circumstantial approach to vulnerability. An often implicit but widely shared assumption in this debate is that vulnerability is morally relevant insofar as it is pathogenic.